Voltage controlled oscillator capable of linear operation at very low frequencies

ABSTRACT

There is disclosed a voltage controlled oscillator (VCO) that receives +V(IN) and −V(IN) control voltages and outputs a VCO output signal having an oscillation frequency determined by the +V(IN) and −V(IN) control voltages. The VCO comprises: 1) a storage capacitor charged linearly by a constant charge current and too discharged linearly by a constant discharge current; 2) a comparator for comparing the storage capacitor voltage to an upper threshold voltage and a lower threshold voltage. The comparator output drops to a negative saturation voltage (−V(SAT)) when the storage capacitor voltage rises above the upper threshold voltage and rises to a positive saturation voltage (+V(SAT)) when the storage capacitor voltage drops below the lower threshold voltage. The VCO also comprises: 3) a constant charge current source for injecting the constant charge current into the storage capacitor when the comparator output rises to the positive saturation voltage; and 4) a constant discharge current source for draining the constant discharge current from the storage capacitor when the comparator output drops to the negative saturation voltage.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention is directed generally to data processors,radio transceivers, and other circuits requiring a stable clockreference and, more specifically, to a voltage controlled oscillator(VCO) capable of linear operation at very low frequencies.

BACKGROUND OF THE INVENTION

[0002] A voltage controlled oscillator (VCO) is a common circuit used togenerate, among other things, clock signals for use in data processors,radio transceivers, and similar circuits. Typically, the VCO is part ofa phase-locked loop (PLL) that uses feedback to provide an accurate andstable clock reference signal for other circuits. The frequency ofoscillations generated by a VCO are controlled by an externally appliedcontrol voltage. Two important parameters in VCO design are linearityand sweep range. Linearity correlates the change in frequency or the VCOoutput to the change in the control voltage. The sweep range is therange of possible frequencies produced by VCO control voltage. A problemfor many voltage controlled oscillators is that there is a trade-offbetween the sweep range and linearity at low frequency. For anyappreciably large sweep range, a conventional VCO typically exhibitsnon-linearity at low frequency.

[0003] Therefore, there is a need in the art for an improved voltagecontrolled oscillator that has improved linearity at low frequency. Moreparticularly, there is a need in the art for a voltage controlledoscillator that has a large sweep range while maintaining high linearityat low frequency.

SUMMARY OF THE INVENTION

[0004] To address the above-discussed deficiencies of the prior art, itis a primary object of the present invention to provide a voltagecontrolled oscillator (VCO) capable of receiving a +V(IN) controlvoltage and a −V(IN) control voltage and outputting a VCO output signalhaving a frequency of oscillation determined by the +V(IN) controlvoltage and the −V(IN) control voltage. According to an advantageousembodiment of the present invention, the VCO comprising: 1) a storagecapacitor capable of being charged linearly by a constant charge currentand discharged linearly by a constant discharge current; 2) a comparatorcapable of comparing a voltage on the storage capacitor to an upperthreshold voltage and a lower threshold voltage, wherein an output ofthe comparator drops to a negative saturation voltage (−V(SAT)) when thestorage capacitor voltage rises above the upper threshold voltage andthe comparator output rises to a positive saturation voltage (+V(SAT))when the storage capacitor voltage drops below the lower thresholdvoltage; 3) a constant charge current source capable of injecting theconstant charge current to the storage capacitor when the comparatoroutput rises to the positive saturation voltage; and 4) a constantdischarge current source capable of draining the constant dischargecurrent from the storage capacitor when the comparator output drops tothe negative saturation voltage.

[0005] According to one embodiment of the present invention, thecomparator output is coupled to the VCO output.

[0006] According to another embodiment of the present invention, theconstant charge current is determined by the +V(IN) control voltage.

[0007] According to still another embodiment of the present invention,the constant discharge current is determined by the −V(IN) controlvoltage.

[0008] According to yet another embodiment of the present invention, theconstant charge current source comprises a PNP-type bipolar junctiontransistor having a base coupled to the +V(IN) control voltage, anemitter coupled to the comparator output via a load resistor, and acollector coupled to the storage capacitor.

[0009] According to a further embodiment of the present invention, theconstant charge current source comprises a NPN-type bipolar junctiontransistor having a base coupled to the −V(IN) control voltage, anemitter coupled to the comparator output via the load resistor, and acollector coupled to the storage capacitor.

[0010] According to a still further embodiment of the present invention,the comparator comprises: 1) an operational amplifier having aninverting input coupled to the storage capacitor; 2) a first resistor(R1) having a first terminal coupled to ground and a second terminalcoupled to a non-inverting input of the operational amplifier; and 3) asecond resistor (R2) having a first terminal coupled to an output of theoperational amplifier and a second terminal coupled to the non-invertinginput of the operational amplifier, wherein the operational amplifieroutput comprises the comparator output.

[0011] According to a yet further embodiment of the present invention,the upper threshold voltage is determined by the no product:

[(R1)/(R1+R2)][+V(SAT)].

[0012] In one embodiment of the present invention, the lower thresholdvoltage is determined by the product:

[(R1)/(R1+R2)][−V(SAT)].

[0013] In another embodiment of the present invention, the constantcharge current is determined by a difference between the positivesaturation voltage and the +V(IN) control voltage and constant dischargecurrent is determined by a difference between the negative saturationvoltage and the −V(IN) control voltage.

[0014] The foregoing has outlined rather broadly the features andtechnical advantages of the present invention so that those skilled inthe art may better understand the detailed description of the inventionthat follows. Additional features and advantages of the invention willbe described hereinafter that form the subject of the claims of theinvention. Those skilled in the art should appreciate that they mayreadily use the conception and the specific embodiment disclosed as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. Those skilled in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the invention in its broadest form.

[0015] Before undertaking the DETAILED DESCRIPTION OF THE INVENTIONbelow, it may be advantageous to set forth definitions of certain wordsand phrases used throughout this patent document: the terms “include”and “comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings,wherein like numbers designate like objects, and in which:

[0017]FIG. 1 illustrates an exemplary processing system in which aclocked circuit receives a clock signal from a phase locked loop (PLL)containing a voltage controlled oscillator in accordance with theprinciples of the present invention;

[0018]FIG. 2 illustrates an exemplary voltage controlled oscillator ingreater detail according to a first embodiment of the present invention;

[0019]FIG. 3 shows the waveforms for the output voltage, V(O), and thecapacitor C1 voltage, V(C), of an exemplary voltage controlledoscillator according to the principles of the present invention;

[0020]FIG. 4 illustrates the frequency-voltage plot of an exemplaryvoltage controlled oscillator according to the principles of the presentinvention; and

[0021]FIG. 5 illustrates the exemplary voltage controlled oscillator ingreater detail according to a second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0022]FIGS. 1 through 5, discussed below, and the various embodimentsused to describe the principles of the present invention in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the invention. Those skilled in the artwill understand that the principles of the present invention may beimplemented in any suitably arranged data processor, radio transceiver,or other circuit requiring a voltage controlled oscillator that operateslinearly at low frequency.

[0023]FIG. 1 illustrates exemplary processing system 100 in whichclocked circuit 125 receives a clock signal from a phase locked loop(PLL) containing voltage controlled oscillator 115 in accordance withthe principles of the present invention. Processing system 100 comprisesphase detector 105, charge pump and loop filter block 110, voltagecontrolled oscillator (VCO) 115, divider circuit 120, and clockedcircuit 125. The PLL comprises phase detector 105, charge pump and loopfilter block 110, VCO 115, and divider circuit 120.

[0024] Clocked circuit 125 is intended to represent generically any typeof circuit component that required a known stable clock referencesignal. Thus, clocked circuit 125 may comprise a data processor, asignal processor, a radio transceiver, an ASIC device, or the like.

[0025] Phase detector 105 detects the phase difference between theoutput of divider circuit 120 and a crystal oscillator reference signaland generates a sequence of pulses in which the pulse-width variesaccording to the magnitude of the phase difference. For example, thecrystal oscillator reference signal may be a 1 MHz signal and thedivider circuit 120 may divide the 8 MHz reference frequency (REF.FREQ.) produced by VCO 115 by a value of N=8 to produce a 1 MHz output.If the two 1 MHz signals are exactly in phase, the pulses produced byphase detector 105 have a predetermined width. The pulses are convertedto a smooth DC VCO control voltage by charge pump and loop filter block110. According to an exemplary embodiment of the present invention, theVCO control voltage may be a differential voltage signal. The VCOcontrol voltage controls the frequency of the output of VCO 115, whichis applied as a clock signal to clocked circuit 125.

[0026] If the 1 MHz frequency of the output of divider circuit 120begins to lag the 1 MHz crystal oscillator (i.e., actual referencecarrier frequency is too low), the pulses produced by phase detector 105increase in width. The wider pulses are converted to a larger VCOcontrol voltage by charge pump and loop filter block 110. The larger VCOcontrol voltage increases the frequency of the output of VCO 115, whichincreases the frequency of the output of divider circuit 120. Similarly,if the 1 MHz frequency of the output of divider circuit 120 begins tolead the 1 MHz crystal oscillator (i.e., actual reference carrierfrequency is too high), the pulses produced by phase detector 105decrease in width. The narrower pulses are converted to a smaller VCOcontrol voltage by charge pump and loop filter block 110. The smallerVCO control voltage decreases the frequency of the output of VCO 115,which no decreases the frequency of the output of divider circuit 120.

[0027] The value of the divider, N, used by divider circuit 120 may beset by clocked circuit 125 or by some external controller (not shown).In some instances, it may be necessary or desirable to operate clockedcircuit 125 at an extremely low frequency or to at least supply a verylow-frequency clock signal (along with one or more high-frequency clocksignals) to clocked circuit 125. To meet this requirement, the presentinvention provides a voltage controlled oscillator that is capable oflinear operation at very low frequency. The highest oscillationfrequency possible depends only on the parasitic capacitances of thecomponents used. As a result, a VCO according to the principles of thepresent invention is useful in applications in which high linearity anda large sweep range are desired.

[0028]FIG. 2 illustrates voltage controlled oscillator 115 in greaterdetail according to a first embodiment of the present invention. Voltagecontrolled oscillator 115 comprises operational amplifier 210, NPN-typebipolar junction transistor 220, PNP-type bipolar junction transistor230, capacitor C1, and resistors R1, R2, and R3. VCO 115 receives adifferential voltage control signal, [+V(IN), −V(IN)], from charge pumpand loop filter block 110 and generates the output voltage V(O).Operational amplifier 210 operates as a comparator and transistors 220and 230 operate as voltage controlled current sources charging capacitorC1.

[0029] The voltage across capacitor C1 depends on the state of thecomparator and the control voltage [+V(IN), −V(IN)] applied at thetransistor base terminals. When the comparator is in positivesaturation, capacitor C1 is charged by a constant current supplied bytransistor 230. When the comparator is in negative saturation,transistor 220 supplies the current to discharge capacitor C1.

[0030]FIG. 3 shows the waveforms for the output voltage, V(O), and thecapacitor C1 voltage, V(C). Operational amplifier 210 operates as acomparator due to positive feedback provided by resistors R1 and R2. Thenon-inverting input of operational amplifier 210 is labeled V+ and theinverting input of operational amplifier 210 is labeled V−. The outputvoltage V(O) swings between its saturation values +V(SAT) and −V(SAT).The output V(O) changes from +V(SAT) to −V(SAT) when V−>βV(SAT), where:

β=R2/(R1+R2).

[0031] The output changes from −V(SAT) to +V(SAT) when V−<−βV(SAT).

[0032] After the circuit reaches a steady state, assume that outputvoltage V(O) of the operational amplifier has just saturated atV(O)=V(SAT). Transistor 230 is in its active region if

V(SAT)−V(IN)≧V(τ)

[0033] where V(τ) is the cut-in voltage of transistor 230. Transistor220 is cut-off because its emitter terminal is at a higher voltage thatits base terminal. As a result,

I(2)=[V(SAT)−V(τ)−V(IN)]/R

[0034] and I(1)=0. Therefore, the current through capacitor C1 is:

I(C)=I(1)+I(2)=I(2).

[0035] The voltage V(C) across capacitor C1 rises linearly and is givenby

V(C)=[I(2)/C]t−βV(SAT)

=[[V(SAT)−V(τ)−V(1)]/RC]t−βV(SAT),

[0036] where t denotes time.

[0037] When V(C)=V−=βV(SAT) after time T1, the output V(O) changes to−V(SAT). Time T1 is computed as follows:

V(C)(T1)=βV(SAT)

[[V(SAT)−V(τ)−V1]/RC]T1−βV(SAT)=βV(SAT)

T1=[2βV(SAT)RC]/[V(SAT)−V(τ)−V(1).   (1)

[0038] After time T1, transistor 230 is cut-off because its base-emitterjunction is reverse biased. However, transistor 220 is in its activeregion if

−V(SAT)+V(IN)>V(τ)

[0039] As a result, I(2)=0 and the current through the capacitor C1 isgiven by

I(C)=I(1)=[−V(SAT)+V(IN)+V(τ)]/R.

[0040] The voltage across capacitor C1 falls linearly and is given by:

V(C)=[I(C)/C](t−T1)+βV(SAT);

=[[−V(SAT)+V(IN)+V(τ)]/RC](t−T1)+τV(SAT)

[0041] At time t−T1=T2, the capacitor C1 voltage V(C)=−βV(SAT), and theoutput switches to +V(SAT), thereby starting a new cycle of oscillation.T2 is found as follows

V(C)(T1+T2)=−βV(SAT)

[[−V(SAT)+V(IN)+V(τ)]/RC]T2+βV(SAT)=−βV(SAT)

T2=[2βV(SAT)RC]/[V(SAT)−V(τ)−V(IN)]  (2)

[0042] The frequency of oscillation, f, is defined as

f=1/[T1+T2]  (3)

[0043] Substituting Equations 1 and 2 in Equation 3, we obtain

f=[V(SAT)−V(τ)−V(1)]/[4βV(SAT)RC]  (4)

[0044] From Equation 4 it is clear that f has a linear relationship withthe input voltage V(IN). As V(IN) is increased toward V(SAT), thefrequency of oscillation approaches 0 Hz.

[0045]FIG. 4 illustrates the frequency-voltage plot of exemplary tovoltage controlled oscillator 115. The plot consists of two regions: 1)a linear region in which the oscillation frequency depends on thecontrol voltage; and 2) a saturation region in which the frequency islimited by parasitic effects such as the slew-rate of operationalamplifier 510.

[0046]FIG. 5 illustrates exemplary voltage controlled oscillator 115 ingreater detail according to a second embodiment of the presentinvention. In the embodiment shown in FIG. 5, metal-oxide-silicon fieldeffect transistors (MOSFET) are used. In FIG. 5, voltage controlledoscillator 115 comprises operational amplifier 510, resistors R1 and R2,inverter 515, N-type transistor 521, P-type transistor 522, N-typetransistor 523, P-type transistor 524, capacitor C5, voltage controlledcurrent (VCC) source 530, and voltage controlled current (VCC) sink 540.VCC source 530 comprises P-type transistor 531, P-type transistor 532,resistor R3, and amplifier 535. VCC source 540 comprises N-typetransistor 541, P-type transistor 542, resistor R4, and amplifier 545.

[0047] Transistors 522 and 523 charge and discharge capacitor C5alternately. Operational amplifier 510 is used a comparator. Thecomparator UTP (Upper Trigger Point) and LTP (Lower Trigger Point)voltages are +βV(SAT) and −βV(SAT), respectively, where β=(R2)/(R1+R2).The output of inverter 515 is the inverse of the output of amplifier 510and clips the swing from between +V(SAT) and −V(SAT) to between VDD(e.g., +5V) and VSS (e.g., −5V).

[0048] When the output of amplifier 510 drops to −V(SAT), the output ofinverter 510, V(O), rises to +5 volts, and transistor 523 is switched ONand transistor 522 is switched OFF. As a result, capacitor C5 dischargesinto VCC sink 540. As capacitor C5 is discharged with a constant currentsource, its voltage drops in a linear fashion with time. When itsvoltage crosses the LTP of the comparator circuit formed by amplifier510, the output of amplifier 510 swings to +V(SAT), and V(O), the outputof inverter 510, drops to −5 volts, switching transistor 523 OFF andswitching transistor 524 ON. Capacitor C5 is then charged with thecurrent supplied by VCC source 530. The oscillations repeat when thecapacitor C5 voltage crosses the UTP of the comparator formed byamplifier 510 and resistors R1 and R2. Transistors 521 and 524 are usedto load transistors 531 and 541, respectively, when transistors 522 and523 are switched OFF. This prevents VCC source 530 and VCC sink 540 fromoperating in non-linear regions.

[0049] In order to ensure that VCC source 530 and VCC sink 540 operateas a voltage controlled constant current source (or sink), it isnecessary that transistors 531 and 532 be matched as closely as possibleand transistors 541 and 542 be matched as closely as possible.Additionally, transistors 531, 532, 541, and 541 must all operate intheir pinch-off regions. Transistors 531 and 532 form a current mirror,since both have the same gate-to-source voltages. Transistors 541 and542 also form a current mirror, since both have the same gate-to-sourcevoltages.

[0050] For example, for transistors 541 and 542, this condition requiresthat:

V(REF)>V(N)−V(TN)   (5)

[0051] where V(N) is the voltage at the gate terminals of transistors541 and 542 with respect to ground, and V(TN) is the threshold voltageof transistors 541 and 542. When the transistors are pinched-off, thedrain current I_(D) through transistor 542 is given by

I _(D) =[V(IN)−V(REF)]/R

=k(W/L)[V(N)−V′−V(TN)]²[1+λ(V′−V(REF))]  (6)

[0052] where W/L denotes the aspect ratio of transistor 542, and V′denotes the output voltage of operational amplifier 545. Equation 6 maybe used to find V′, which must be in the linear region of operationalamplifier 545. The same drain current I_(D) flows in transistor 541. Thecurrent in transistor 541 is the constant discharging current thatdischarges capacitor C5 in a linear fashion. Similarly, the constantcurrent in transistor 532 is mirrored in transistor 531. The current intransistor 531 is the constant charging current that charges capacitorC5 in a linear fashion.

[0053] Although the present invention has been described in detail,those skilled in the art should understand that they can make variouschanges, substitutions and alterations herein without departing from thespirit and scope of the invention in its broadest form.

What is claimed is:
 1. A voltage controlled oscillator (VCO) capable ofreceiving a +V(IN) control voltage and a −V(IN) control voltage andoutputting a VCO output signal having a frequency of oscillationdetermined by said +V(IN) control voltage and said −V(IN) controlvoltage, said VCO comprising: a storage capacitor capable of beingcharged linearly by a constant charge current and discharged linearly bya constant discharge current; a comparator capable of comparing avoltage on said storage capacitor to an upper threshold voltage and alower threshold voltage, wherein an output of said comparator drops to anegative saturation voltage (−V(SAT)) when said storage capacitorvoltage rises above said upper threshold voltage and said comparatoroutput rises to a positive saturation voltage (+V(SAT)) when saidstorage capacitor voltage drops below said lower threshold voltage; aconstant charge current source capable of injecting said constant chargecurrent to said storage capacitor when said comparator output rises tosaid positive saturation voltage; and a constant discharge currentsource capable of draining said constant discharge current from saidstorage capacitor when said comparator output drops to said negativesaturation voltage.
 2. The voltage controlled oscillator (VCO) as setforth in claim 1 wherein said comparator output is coupled to said VCOoutput.
 3. The voltage controlled oscillator (VCO) as set forth in claim2 wherein said constant charge current is determined by said +V(IN)control voltage.
 4. The voltage controlled oscillator (VCO) as set forthin claim 3 wherein said constant discharge current is determined by said−V(IN) control voltage.
 5. The voltage controlled oscillator (VCO) asset forth in claim 4 wherein said constant charge current sourcecomprises a PNP-type bipolar junction transistor having a base coupledto said +V(IN) control voltage, an emitter coupled to said comparatoroutput via a load resistor, and a collector coupled to said storagecapacitor.
 6. The voltage controlled oscillator (VCO) as set forth inclaim 5 wherein said constant charge current source comprises a NPN-typebipolar junction transistor having a base coupled to said −V(IN) controlvoltage, an emitter coupled to said comparator output via said loadresistor, and a collector coupled to said storage capacitor.
 7. Thevoltage controlled oscillator (VCO) as set forth in claim 6 wherein saidcomparator comprises: 1) an operational amplifier having an invertinginput coupled to said storage capacitor; 2) a first resistor (R1) havinga first terminal coupled to ground and a second terminal coupled to anon-inverting input of said operational amplifier; and 3) a secondresistor (R2) having a first terminal coupled to an output of saidoperational amplifier and a second terminal coupled to saidnon-inverting input of said operational amplifier, wherein saidoperational amplifier output comprises said comparator output.
 8. Thevoltage controlled oscillator (VCO) as set forth in claim 7 wherein saidupper threshold voltage is determined by the product:[(R1)/(R1+R2)][+V(SAT)].
 9. The voltage controlled oscillator (VCO) asset forth in claim 7 wherein said lower threshold voltage is determinedby the product: [(R1)/(R1+R2)][−V(SAT)].
 10. The voltage controlledoscillator (VCO) as set forth in claim 6 wherein said constant chargecurrent is determined by a difference between said positive saturationvoltage and said +V(IN) control voltage and constant discharge currentis determined by a difference between said negative saturation voltageand said −V(IN) control voltage.
 11. A processing system comprising: aclocked circuit capable of operating at very low frequency using anexternal clock signal; and a phase-locked loop (PLL) coupled to saidclocked circuit capable of supplying said external clock signal withhigh linearity at very low frequency, said PLL comprising: a frequencydivider circuit for dividing a frequency of said external clock signalby N; a phase detector capable of detecting a phase difference between afrequency divided output of said frequency divider circuit and an inputreference signal and generating therefrom a phase difference signal; acharge pump and loop filter circuit capable of converting said phasedifference signal to a +V(IN) control voltage and a −V(IN) controlvoltage; and a voltage controlled oscillator (VCO) capable of receivingsaid +V(IN) control voltage and said −V(IN) control voltage andoutputting a VCO output signal having a frequency of oscillationdetermined by said +V(IN) control voltage and said −V(IN) controlvoltage, said VCO comprising: a storage capacitor capable of beingcharged linearly by a constant charge current and discharged linearly bya constant discharge current; a comparator capable of comparing avoltage on said storage capacitor to an upper threshold voltage and alower threshold voltage, wherein an output of said comparator drops to anegative saturation voltage (−V(SAT)) when said storage capacitorvoltage rises above said upper threshold voltage and said comparatoroutput rises to a positive saturation voltage (+V(SAT)) when saidstorage capacitor voltage drops below said lower threshold voltage; aconstant charge current source capable of injecting said constant chargecurrent to said storage capacitor when said comparator output rises tosaid positive saturation voltage; and a constant discharge currentsource capable of draining said constant discharge current from saidstorage capacitor when said comparator output drops to said negativesaturation voltage.
 12. The processing system as set forth in claim 11wherein said comparator output is coupled to said VCO output.
 13. Theprocessing system as set forth in claim 12 wherein said constant chargecurrent is determined by said +V(IN) control voltage.
 14. The processingsystem as set forth in claim 13 wherein said constant discharge currentis determined by said −V(IN) control voltage.
 15. The processing systemas set forth in claim 14 wherein said constant charge current sourcecomprises a PNP-type bipolar junction transistor having a base coupledto said +V(IN) control voltage, an emitter coupled to said comparatoroutput via a load resistor, and a collector coupled to said storagecapacitor.
 16. The processing system as set forth in claim 15 whereinsaid constant charge current source comprises a NPN-type bipolarjunction transistor having a base coupled to said −V(IN) controlvoltage, an emitter coupled to said comparator output via said loadresistor, and a collector coupled to said storage capacitor.
 17. Theprocessing system as set forth in claim 16 wherein said comparatorcomprises: 1) an operational amplifier having an inverting input coupledto said storage capacitor; 2) a first resistor (R1) having a firstterminal coupled to ground and a second terminal coupled to anon-inverting input of said operational amplifier; and 3) a secondresistor (R2) having a first terminal coupled to an output of saidoperational amplifier and a second terminal coupled to saidnon-inverting input of said operational amplifier, wherein saidoperational amplifier output comprises said comparator output.
 18. Theprocessing system as set forth in claim 17 wherein said upper thresholdvoltage is determined by the product: [(R1)/(R1+R2)][+V(SAT)].
 19. Theprocessing system as set forth in claim 17 wherein said lower thresholdvoltage is determined by the product: [(R1)/(R1+R2)][−V(SAT)].
 20. Theprocessing system as set forth in claim 16 wherein said constant chargecurrent is determined by a difference between said positive saturationvoltage and said +V(IN) control voltage and constant discharge currentis determined by a difference between said negative saturation voltageand said −V(IN) control voltage.